1. Field of the Invention
The field of the invention generally relates to techniques for precisely measuring the flow rates of gases and liquids and mixtures thereof. In particular, the field of the invention relates to a flow measurement device positioned in a flow path for measuring liquids and gases, or a mixture thereof that achieves a previously unattainable degree of precision. Positioning of the flow measurement device in the flow path eliminates the complexity and inaccuracies inherent in a conventional bypass structure and enables precise real time control of process parameters.
2. Background of Related Art
Accurate flow rates of gases and liquids are of wide-ranging interest in our economy. In particular, as semiconductor feature sizes shrink to atomic scales, it is essential that mass flow parameters be maintained with tighter control than was previously possible. Sophisticated applications are common in semiconductor manufacturing, pharmaceuticals and optics among other areas of commerce. Small process variations, too small to be accommodated by conventional mass flow devices, can result in unacceptable process variations when feature sizes are measured with respect to nanometers.
Two concepts are commonly in use for sophisticated applications requiring precise mass flow rates. These two concepts are commonly referred to as “bypass or capillary” mass flow meters (MFM) and “bubbler” mass flow metering devices. Although there are a number of variations of these conventional mass flow meters, they all share a number of problems and limitations. These problems and limitations include, inherent inaccuracies, inherent process variations, maintenance difficulties, cleaning difficulties and inefficient size.
the conventional “bypass or capillary” MFM (FIG. 1) makes use of the change in a temperature differential caused by mass flow through the bypass structure. Typical conventional MFM's of this design are disclosed in U.S. Pat. No. 2,729,976 (Laub), U.S. Pat. No. 3,938,384 (Blair) and U.S. Pat. No. 4,487,062 (Olin). When the temperature differential (Temperature, T2 greater than Temperature T1) is measured by electronic temperature sensors, the flow signal becomes an electrical signal that is highly useful for automated process control. The MFM can be small, accurate and minimally contaminating to the process of interest.
However, the conventional “bypass or capillary” MFM has subtle limitations that increasingly limit its suitability for sophisticated processing. Heat transfer within the sensor itself changes due to many factors, such as intimacy of the heating element bond to electrical insulation, electrical insulation bond to capillary, heat transfer through capillary wall, deposits built up inside capillary, and heat transfer from edge to the center of capillary flow. These changes in heat transfer characteristics affect the correlation between heater energy and temperature differential and reduce the precision of the MFM.
The capillary part of this traditional MFM is a very small flow tube intended to minimize temperature difference from an edge to the center of flow. Such temperature difference is undesirable because it introduces an uncertainty in the amount of fluid being heated and thus an uncertainty in the amount of fluid flow. Typically, only a small portion of the total flow passes through the capillary. Thus, a large portion of the flow must pass through a calibrated by-pass without contributing to the flow measurement at all. An assumption must be made that flow proportions through the capillary and by-pass stay the same regardless of fluid composition, process temperature, process pressure and overall process flow rates. This assumption introduces an inherent inaccuracy into all “bypass or capillary” MFMs.
The above limitations apply to gaseous flow. For liquid flow, additional complications impose limitations on the accuracy of conventional methods of mass flow measurement. Bubbling, voiding or boiling can occur as the liquid passes through the capillary. This can result from increases in temperature associated with heating by the heater, pressure drops and directional changes, among other reasons, as the fluid passes into and through the capillary. These effects can result in a change in phase of liquid to vapor or the emergence of dissolved gases from the fluid. The introduction of two phases into the capillary significantly and unpredictably alters the correlation between heater energy and temperature differential. Under these circumstances, the flow signals become totally unreliable.
Because of conventional MFM difficulties in handling liquids, bubblers (FIG. 2) are sometimes used as an alternative means of metering mass flow rates. Typical conventional bubbler designs are disclosed in U.S. Pat. No. 4,134,514 (Schumacher) , U.S. Pat. No. 4,140,735 (Schumacher) and U.S. Pat. No. 4,436,674 (McMenamin). Sophisticated applications require the use of gaseous ingredients to achieve the precise mass flow rates necessary in applications where atomic scales are important. Therefore, a mass flow metering device (bubbler) starting with a liquid ingredient requires a change of state; i.e. controlled evaporation to a gaseous state. The amount of liquid used is frequently very small which makes accurate measurement difficult. When the liquid is converted to a gas, a volume expansion of about 1000 times occurs which then makes measurement easier.
A bubbler, (FIG. 2) is a container of liquid kept at a fixed temperature by the heater (with appropriate controls, enclosures, etc.). An inlet gas bubbles into the liquid, mixing with evaporant. Above the liquid level, the proportion of gas to evaporant is directly related to the liquid temperature and the container pressure. If container pressure surges higher, the evaporant proportion will be reduced. If container temperature is slightly cooler at the top, condensation will occur, also lowering evaporant proportion. If bubbling action is too vigorous, atomization can occur, producing aerosols that represent increased proportion. The liquid level must be kept constant or the thermal and evaporative characteristics of the bubbler change.
To keep a constant level as evaporation removes liquid, a level sensor must be provided. The level sensor signals additional liquid delivery through the refill port. Of course, the refill liquid must be the same temperature or the bubbler characteristics will change. If all goes well, a specific concentration of evaporant in inlet gas passes through the outlet to the process area. The outlet region must be kept sufficiently hot or condensation will occur, lowering evaporant concentration.
Obviously, accurate control of a bubbler is no simple task. For some processes, several liquids may be required. Each liquid requires its own bubbler. Even if the liquids can be freely mixed without altering their properties, the bubbling process constitutes distillation. Evaporation would distort the liquid proportions unless all liquids had exactly the same evaporation characteristics.
Fluid flow measurement is becoming critical to increasing operational efficiency of many commercial processes, such as in semiconductor process control. As semiconductor device feature sizes shrink to atomic dimensions, improved in line process measurements that do not introduce inaccuracies or assumptions are needed to control fluid flow. Therefore, what is needed is a new method for directly measuring fluid flow that provides precise in situ measurement of fluid flow rate for enabling real time control of process parameters without error.